A method to cool, heat, or dry an article such as foodstuffs as it is transferred on a belt conveyor in an oblong housing has been disclosed in prior arts, in which a gas jet is allowed to impinge on the article. In those prior arts, a chilled air is spouted from hole nozzles or slit nozzles perpendicular to the surface of the belt and a thin stream layer is formed on the surface of the article placed on the belt owing to Coanda effect, by which heat transfer between the chilled air and the surface of the article is increased.
For example, a transfer type freezer is disclosed in Japanese Patent Application Publication No. 8-507596 (prior art 1), in which a tunnel is provided in a housing having an entrance and exit opening for a conveyor belt to surround the conveyor belt with the upper wall, two side walls extending along the transfer direction of the conveyor belt, and the bottom wall of the tunnel, the inside of the tunnel is vacuumized by a gas sucking means to form gas circulation in the housing such that the gas in the housing flows through holes formed in said upper wall into the tunnel, exits from the tunnel through an opening formed in said bottom wall, and again flows into the tunnel through said holes, thus a gas jet stream is formed to impinge against the article placed on the conveyor belt.
FIG. 7 is a schematic representation of the equipment of said prior art 1 for explaining the principle of operation thereof, and FIG. 7(a) is a partial enlarged detail of FIG. 7. The equipment comprises a housing 01, in which a tunnel 02 is provided to surround a treating zone 03. The gas in the treating zone 03 is sucked by a sucking means 04 to vacuumize the treating zone. A conveyor belt 05 transfers foodstuffs 06 through the tunnel.
In FIG. 7, holes are bored only in the upper wall 07 of the tunnel 02, the upper and lower surfaces of the upper wall being flat and nozzles being formed without no part protruding from the upper and lower surfaces of the upper wall (see FIG. 7a). In a preferable embodiment, holes are bored also in the bottom wall of the tunnel 02.
During operation of the equipment, the sucking means 04 vacuumize the inside space of the tunnel 02 so that the air in the housing is sucked through the nozzles 8 in the upper wall 07 and air jets 09 are formed to impinge against the conveyor belt 05 and the foodstuffs 06 placed thereon.
In Japanese Laid-Open Patent Application No. 11-63777 (prior art 2) is disclosed a transfer type freezer in which articles such as foodstuffs are placed on a conveyor belt passing through an oblong housing, a number of slits to produce jet streams are formed in an upper and lower plates facing the conveyor belt, the jet streams of chilled air impinge on the articles to cool or freeze them, and the chilled air that has cooled the articles is exhausted in one direction.
With said prior art 1, a vacuum chamber is necessary to be provided in order to vacuumize the tunnel surrounding the conveyor belt. For this purpose, a large fan is required as a sacking means and large power is needed to drive the fan. Further, as air jet streams are formed by sacking air by the fan through the holes bored in the upper wall, the jet flows lack in directionality and tend to diffuse or proliferate. Therefore, the air jet streams do not necessarily impinge on the articles at high speed resulting in low heat transfer between the air and the surface of the articles.
Further, air exit openings are provided below the conveyor belt at certain intervals, so nozzles can not be provided where the exit openings are provided under the conveyor belt, resulting in decreased cooling efficiency. There is no air exit opening above the conveyor belt and the air introduced into the tunnel through the nozzles (holes) bored in the upper wall must flow through the holes bored in the conveyor belt to be exhausted from the tunnel. Therefore, the conveyor belt must have holes and a conveyor belt not having holes can not be used. Therefore, loading density of the articles transferred on the belt is limited, because there must be left on the belt space not occupied by the articles in order to allow the air introduced into the tunnel through the nozzles (holes) to flow downward through the holes of the conveyor belt.
According to the prior art 2, chilled air jets impinge against both upper surface and undersurface of the conveyor belt and then the chilled air is exhausted in one direction perpendicular to the belt transfer direction, the air jets tend to incline to said exhausting direction, which causes reduction in heat transfer coefficient between the chilled air and the surface of the article on the belt. Further, the space in which the conveyor belt passes through is narrow and inconvenient to clean. Therefore, it is necessary to provide a number of cleaning nozzles, and the installation becomes complicated resulting in increased cost.